To prepare regulation on green gas emission caused by a climate change and exhaust of fossil fuel, some studies about application of ammonia (17.6 wt % H2) that is a hydrogen-storing body to fuel cells, fuel for vehicles, or the like, as a substitute for fuel. Ammonia, a carbon-free energy carrier, is liquefied at 21° C. under 7.86 bar and thus is convenient to store and transport as compared to hydrogen, and emits water and nitrogen only as shown in the following Reaction Scheme 1 upon the complete combustion. In addition, ammonia has a high energy density per volume (HHV NH3: 13.6 GJ/m3) similar to that of fossil fuel (HHV LPG: 19 GJ/m3, HHV CNG: 10.4 GJ/m3) (Non-Patent Document 1).4NH3(g)+3O2(g)→2N2(g)+6H2O(g), ΔH=−1266 kJ/mol  [Reaction Scheme 1]
The most general method for producing ammonia is the Haber-Bosch process using hydrogen and nitrogen as starting materials and carried out under a high pressure (150-250 bar) at a high temperature (400-500° C.) in the presence of an iron or ruthenium catalyst as shown in the following Reaction Scheme 2. The reaction consumes a significantly large amount of energy of about 34.4 GJ/ton NH3 and causes a problem in that it emits a large amount of greenhouse gas corresponding to 1.8 ton CO2/ton NH3 due to the fossil fuel used for supplying the energy.N2+3H2→2NH3+92.2 kJ  [Reaction Scheme 2]
To overcome the above-mentioned problem of the Haber-Bosch process, an electrochemical method for ammonia synthesis using an ion conductive oxide electrolyte has been suggested. Active studies have been conducted about an electrochemical method for ammonia synthesis from water and nitrogen using an electrolyte (Non-Patent Document 2).
An electrochemical method for ammonia synthesis undergoes a series of steps as shown in the following Reaction Scheme 3, and includes reaction (3-1) in which water is decomposed at an anode to be divided into protons and electrons and reaction (3-2) in which the protons and electrons reduce nitrogen molecules to produce ammonia. The final products of the electrochemical method for ammonia synthesis merely include ammonia and oxygen, and thus the method avoids carbon emission advantageously.[Reaction Scheme 3]Anode reaction: 3H2O→6H++3/2O2+6e−  (3-1)Cathode reaction: N2+6H++6e−→2NH3  (3-2)
The main limiting reaction in the electrochemical ammonia synthesis reactions is the step of reducing nitrogen molecules into ammonia, which proceeds at the cathode. This results from the dissociation of strong triple bond of a nitrogen molecule. In the cathode reaction, competitive hydrogen-generating reaction occurs instead of nitrogen reduction in the presence of protons. In fact, it is known that the current efficiency is less than 1% when using a water electrolysis-based system (Non-Patent Document 3).
Therefore, according to the present disclosure, a single-crystalline metal is used as a catalyst for electrochemical ammonia synthesis. It has been found that use of the catalyst provides a method for ammonia synthesis having improved ammonia production yield and synthesis rate through the reaction of nitrogen and protons. The present disclosure is based on this finding.